Of course the shorters have an endless supply! 11.5% of every...

Of course the shorters have an endless supply! 11.5% of every legal wage in Australia [up to certain limits] goes into Super and at least one third of that goes into the ASX. It's a broad torrent of supply to borrow from at marginal cost to short with. Super money has to go in long, and by mandate has to go into the ASX200 or sometimes the ASX300.
WC and HD are on the wrong path even discussing best execution, IMO. You saving a poofteenth on your buys is neither here nor there, in the bigger picture. In the time it takes between you pushing the button on your phone until execution, some algo bot has scuttled off under the skirting boards with half the difference anyway, most probably.